Romans 2:25, 28-29

If you follow the law, your circumcision has meaning. But if you break the law, it is as if you were never circumcised.

They can do this because a person is not a true Jew if he is only a Jew in his physical body; true circumcision is not only on the outside of the body. A person is a Jew only if he is a Jew inside; true circumcision is done in the heart by the Spirit, not by the written law. Such a person gets praise from God rather than from people.